1. Field of the Invention
The present invention relates to a color combining optical system for combining three color lights, and a projection-type display optical system and a projection-type image display apparatus using the same color combining optical system.
2. Description of the Related Art
In a conventional liquid crystal projector, by improving light utilization efficiency by shortening the arc length of a lamp to illuminate a liquid crystal display element as much as possible and/or by heightening the aperture ratio of a liquid crystal display element, a high-brightness projected image has been realized.
In the present condition, some products using a 0.7-inch small-sized transmissive liquid crystal display element have a brightness above 1000 ANSI lm. Moreover; simultaneously with a high brightness, downsizing of a liquid crystal projector has also been realized.
A conventional liquid crystal projector has an optical system as proposed in Japanese Patent Laid-Open No. 2001-290010 (corresponding to U.S. patent application Publication No. 2002-003668), for example. Namely, on the side of illumination of an illuminating light of transmissive liquid crystal display elements, which modulate light separated into red, green, and blue, respectively, an entrance-side polarizing plate as a polarizer and, on the side of emergence of a color combining optical system for combining the respective color lights modulated by the liquid crystal display elements, an exit-side polarizing plate as an analyzer are arranged so that their polarizing directions are orthogonal to each other. When the liquid crystal display element displays black, since a light transmitted through the liquid crystal display element is absorbed in the exit-side polarizing plate, black can be displayed.
In greater detail, respective color illuminating lights of blue, green, and red are, after being transmitted through entrance-side polarizing plates corresponding to the respective colors, made incident into liquid crystal display elements corresponding to the respective colors and modulated. Then, the respective color image lights modulated by the liquid crystal display elements are, after being transmitted through exit-side polarizing plates, combined by a dichroic prism and projected in an enlarged manner by a projection lens onto a screen. As such, a color combining optical system for combining three color image lights is composed of the polarizing plates and the dichroic prism.
In addition, in Japanese Patent Laid-Open No. H07(1995)-306405, proposed is an optical system which employs polarization beam splitters in place of exit-side polarizing plates as described above and reflects, when displaying black, a light transmitted through a crystal display element in a direction not to proceed toward a projection lens. Furthermore, the optical system proposed in the above-described Japanese Patent Laid-Open No. H07(1995)-306405 employs polarization beam splitters in place of the entrance-side polarizing plates as a polarizer and the exit-side polarizing plate as an analyzer and is further separately provided with a color combining member to carry out a color combination.
However, in the liquid crystal projector as proposed in the above-described Japanese Patent Laid-Open No. 2001-290010, if, for example, power consumption of a lamp is raised to increase the illuminating light amount so as to realize a high brightness liquid crystal projector, the amount of light which must be absorbed by the exit-side polarizing plate is increased when black is displayed, therein arises a thermal problem such that performance of the exit-side polarizing plate is deteriorated.
This problem has been solved by spraying cooling air onto the polarizing plates by a fan or other methods, however, if the cooling air is strongly sprayed onto the polarizing plate, a sound during spraying the cooling air and the noise of the fan are increased.
On the other hand, in the liquid crystal projector as proposed in Japanese Patent Laid-Open No. H07(1995)-306405, since polarization beam splitters are used as a polarizer and an analyzer, even if the illuminating light is increased to brighten the liquid crystal projector, no thermal problem occurs, however, a color combination of the three colors must be carried out by a polarization beam splitter for a color combination, which is provided separately from the polarization beam splitter as an analyzer, and a dichroic prism.
Accordingly, back focal distance of the projection lens is extended, and accordingly the projection lens becomes large, therefore, the whole liquid crystal projector is increased in size.